1. Field of the Invention
This invention relates to granular water soluble plant foods and their controlled release as nutrients. More particularly, it relates to attrition resistant controlled release fertilizer particles comprising a central mass of water soluble fertilizer compounds containing amino groups, a base coating of substituted ureas chemically bonded to the central mass, and a water insoluble sealing layer chemically bonded to the base coating with the sealing layer formed by the reaction of organic polyols with excess polyfunctional isocyanates from the base coat substituted urea formation.
2. Description of the Prior Art
Rapid release of water soluble plant food in soils containing plants may cause phytotoxicity to the plants, and/or the early depletion of the plant food by leaching. Plant food researchers for a long time have attempted to develop plant food particles which release nutrients at a rate about equal to their uptake by plants to minimize phytotoxicity and maximize plant food use efficiency. Improved release control has been achieved primarily with nitrogen in the form of urea, by substantially reacting it with aldehydes to form insoluble products such as ureaform, which must chemically decompose in the soil before the nitrogen becomes available for utilization by plants. Another method consists of physically coating fertilizer granules with solidified water insoluble melts. Various materials have been used as coatings, including sulfur, paraffin waxes, and plastics. These coatings must be broken down by erosion, internal vapor pressure, microbes, or attrition before the contained nutrients become available. Unfortunately, attrition in producing, storing, handling and applying the coated products prematurely breaks down the coatings causing substantial loss of release control, phytotoxic problems, and excessive nutrient leaching.
Silvio Vargiu et al in U.S. Pat. No. 4,120,685 illustrates an improved urea-aldehyde solid reaction product having properties of slow nitrogen release. Even with his improvements Vargiu obtained a nitrogen availability of about 60 percent. The remaining nitrogen is simply not available for plant uptake in a reasonable time period.
Steven Belak in U.S. Pat. No. 3,232,739 showed that free urea incorporated in polyurethane foam is slowly released into soil over a long period of time by the leaching action of water. He was able to use between 10 and 60 percent urea, but when his product contained 10 percent or more urea more than 30 percent of the urea was leached in 24 hours, indicating a nitrogen release rate high enough to cause phytotoxic problems when substantial amounts of urea fertilizer was applied.
Andrew Bozzelli et al in U.S. Pat. No. 3,252,786 disclose a solid controlled release fertilizer composition containing up to 80 percent of solid urea dispersed in petroleum wax blended with wood rosins and other wood rosin related polymers. Unfortunately even with 40 percent wax-rosin phase and only 60 percent urea, 10 percent or more of the urea was dissolved in 24 hour water soak tests. It was necessary that the urea be heated and dispersed in the molten wax and then molded and cooled to form pellets. Attrition with this type of coated particle is a problem because there is no real bond between the urea and the wax and the release control depends upon the small particles of urea being imbedded in a large amount of wax.
Louis Hansen advanced the art in U.S. Pat. No. 3,259,482 by disclosing a delayed release fertilizer containing a core of fertilizer encapsulated or coated with a plurality of water insoluble coatings composed of a mixture of epoxidized soybean oil and a polyester curing agent. At the time of use the materials are combined and the resultant mixture is spread in film form over the fertilizer granule. It is necessary to dry and cure the Hansen product and the heat and escaping vapors open pores for subsequent water intrusion into the soluble fertilizer. Coating adhesion problems are encountered because there is no chemical bond with the central fertilizer mass or particle.
A further disclosure by Hansen in U.S. Pat. No. 3,264,089 teaches that a slow-release fertilizer can be prepared by encapsulating or coating fertilizer granules with well known urethane resins. The polyurethane polymers were generally diluted with solvents and stored until they were applied. A plurality of coatings was required, each amounting to about 0.25 to 3 percent by weight, to give a total coating weight of between 3 and 15 percent. Each successive coating, or layer, must be dried to gelation so that it is not loosened by subsequent solvent wet coating. Hansen found it helpful to precoat the fertilizer granules with a drying oil and a fine clay before application of his urethane resin, but did not chemically react the central fertilizer mass with a base coating material to produce a stable chemical bond. He taught a physical seal of a plurality of water insoluble coatings over the soluble fertilizer granule. Escaping solvent causes bubbles and holes in this type of coating and creates routes for excess water penetration.
Sulfur coated fertilizer, particularly urea, is a widely used form of controlled release plant food nutrients. Glenn Blouin et al disclosed an improved sulfur coated granule process in U.S. Pat. No. 3,342,577, wherein a plurality of layers of molten sulfur is applied to fertilizer granules. An oil based sealer, such as polyethylene plastic dissolved in light petroleum oil, is required to produce slow nutrient dissolution rates and a conditioner, such as diatomaceous earth, is required to eliminate stickiness. Sulfur coated urea is subject to appreciable attrition and cracking when handled in commercial fertilizer systems. Temperature variations cause coating cracking and increased nutrient release rates. The large amounts of sulfur coating required to prevent rapid leaching of nutrients cause the occlusion of some of the nutrients so that they are not released in a useful time period.
It has long been an object of those skilled in the art to produce controlled release fertilizers having substantial resistance to attrition from shipping, handling and application, by applying economically small amounts of coating material.
It is therefore an object of this invention to provide new coated fertilizer particles which release soluble fertilizer nutrients in a slow and controlled manner even with the use of small amounts of coating material.
It is another object to provide coated fertilizer particles with coatings so securely bonded to the soluble fertilizer that little, or no, attrition occurs and release control is maintained even when the coated particles receive severe vibration, and abrasion through repeated rough handling.
It is a further object to provide an effective method for preparing these attrition resistant controlled release coated fertilizer particles.